As a type of steam turbine, steam turbines having a casing, a shaft body (rotor) that is rotatably installed inside the casing, a plurality of turbine vanes that are fixedly disposed on an inner circumference of the casing, and a plurality of turbine blades that are radially installed on the shaft body on a downstream side of the plurality of turbine vanes have been known. In the case of an impulse turbine among these steam turbines, pressure energy of steam is converted into velocity energy by the turbine vanes, and the velocity energy is converted into rotating energy (mechanical energy) by the turbine blades. Further, in the case of a reaction turbine, the pressure energy is converted into velocity energy even inside the turbine blades, and into rotating energy (mechanical energy) by a reaction force with which the steam is spouted out.
In this type of steam turbine, radial clearance is formed between a tip portion of the turbine blade and the casing surrounding the turbine blade to form a flow passage of the steam. Further, the radial clearance is also formed between the tip portion of the turbine vane and the shaft. However, leakage steam passing through the clearance of the tip portion of the turbine blade on the downstream side does not offer a rotating force to the turbine blade. Further, leakage steam passing through the clearance of the tip portion of the turbine vane on the downstream side hardly offers a rotating force to the downstream turbine blade, because the pressure energy of steam is not converted into the velocity energy by the turbine vane. Accordingly, to improve performance of the steam turbine, it is necessary to reduce the amount of the leakage steam passing through the clearance.
In Patent Literature 1 below, there is a proposal for a structure in which the tip portion of the turbine blade are provided with step part whose heights are gradually increased from the axial upstream side to the downstream side, and the casing is provided with seal fins having clearance with respect to the step part.
With this configuration, a leakage flow passing through the clearance of the seal fins collides with end edges of the step part which form step faces of the step part, and increases flow resistance. Thereby, the leakage flow rate is reduced.